ok, well we had to get that out of the way first being one of the most common problems on these.

Now....
in your description you say only in fwd direction it 'dumps' the brake valve? when this happens does the truck stop abruptly?

One other thing... is this truck equipt with manual steering? or electronic steering?
If electronic steering there should be a small steering control board mounted on the top plate above the battery along with the Vehicle Manager. Look on this steer control and there should be a small LED display. When the truck goes into this condition look at that LED and see if it is showing any numbers on it. Generally in normal condition it will just be cycling the LED segments or just be blank... if there are any issues with that it will show a number on it.